This invention relates to a roll electrode of a seam welding machine and a method of operating the same.
A prior art roll electrode comprises an internal stator and a rotor mounted thereon and mercury is filled in a space defined between the stator and the rotor. Such construction is disclosed, for example, in U.S. Pat. No. 3,501,611 dated Mar. 17, 1970.
However, the mercury causes various problems as follows. For example, since the wetting property of mercury is small the contact resistance between it and the surrounding metals is high. Moreover, mercury forms amalgam with surrounding metals, thus increasing resistance. In addition, the poisonous property of mercury is not advantageous especially in a seam welder for manufacturing cans adapted to contain foodstuffs.
However, as it is extremely difficult to perfectly prevent leakage of mercury from the roll electrode various attempts have been made to obviate the use of mercury. For example, liquid metals have been developed. However, presently available liquid metal which is stable at room temperature and not affected by air or moisture is limited to Ga-In alloys. However, the solidifying point of these alloys is higher than that of mercury and the volume thereof increases when they solidify so that when these alloys are substituted for mercury in the roll electrode of the seam welder, they solidify to fracture the roll electrode when the temperature decreases below 5.degree. C.